16 research outputs found
Surface Reconstructions in Molecular Beam Epitaxy of SrTiO3
We show that reflection high-energy electron diffraction (RHEED) can be used
as a highly sensitive tool to track surface and resulting film stoichiometry in
adsorption-limited molecular beam epitaxy of (001) SrTiO3 thin films. Even
under growth conditions that yield films with a lattice parameter that is
identical to that of stoichiometric bulk crystals within the detection limit of
high-resolution x-ray diffraction (XRD), changes in surface reconstruction
occur from (1x1) to (2x1) to c(4x4) as the equivalent beam pressure of the Ti
metalorganic source is increased. These surface reconstructions are correlated
with a shift from mixed SrO/TiO2 termination to pure TiO2 termination. The
crossover to TiO2 surface termination is also apparent in a phase shift in
RHEED oscillations observed at the beginning of growth. Comparison with prior
results for carrier mobilities of doped films shows that the best films are
grown under conditions of a TiO2-saturated surface [c(4x4) reconstruction]
within the XRD growth window.Comment: Accepted for publication in Applied Physics Letter
Probing the metal-insulator transition of NdNiO3 by electrostatic doping
Modulation of the charge carrier density in a Mott material by remote doping
from a highly doped conventional band insulator is proposed to test theoretical
predictions of band filling control of the Mott metal-insulator transition
without introducing lattice distortions or disorder, as is the case for
chemical doping. The approach is experimentally tested using ultrathin (2.5 nm)
NdNiO3 films that are epitaxially grown on La-doped SrTiO3 films. We show that
remote doping systematically changes the charge carrier density in the NdNiO3
film and causes a moderate shift in the metal-insulator transition temperature.
These results are discussed in the context of theoretical models of this class
of materials exhibiting a metal-insulator transition.Comment: The article has been accepted by Applied Physics Letters. After it is
published, it will be found at http://apl.aip.org
Limitations to the room temperature mobility of two- and three-dimensional electron liquids in SrTiO 3
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La-doped SrTiO3 films with large cryogenic thermoelectric power factors
The thermoelectric properties at temperatures between 10 K and 300 K of La-doped SrTiO3 thin films grown by hybrid molecular beam epitaxy (MBE) on undoped SrTiO3 substrates are reported. Below 50 K, the Seebeck coefficients exhibit very large magnitudes due to the influence of phonon drag. Combined with high carrier mobilities, exceeding 50 000 cm2 V-1s-1 at 2 K forthe films with the lowest carrier densities, this leads to thermoelectric power factors as high as 470 µWcm-1K-2. The results are compared with other promising low temperature thermoelectric materials and discussed in the context of coupling with phonons in the undoped substrate